Transistor Switch Example

The base resistor is chosen small enough so that the base current drives the transistor into saturation.

In this example the mechanical switch is used to produce the base current to close the transistor switch to show the principles. In practice, any voltage on the base sufficient to drive the transistor to saturation will close the switch and light the bulb.

NPN Common Emitter Switch

For switching currents less than an ampere, the transistor switch can be used. Instead of a mechanical switch in the base circuit, an op-amp could be used. When the switch
is open, no current
flows in the base so
the collector current
is cut off. The resistor
RB must be small
enough to drive the
transistor to
saturation so that
most of the voltage
Vcc appears across
the load.

In a configuration where the output is taken below the load resistor, this kind of switch can function as an inverting buffer in digital circuits.

NPN Emitter Follower Switch

Though more commonly in the common emitter configuration, the emitter follower can also be used for switching. The resistor
RB must be small
enough to drive the
transistor to
saturation so that
most of the voltage
Vcc appears across
the load.